Rich Freeman on 20 Aug 2011 14:08:54 -0700

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Re: [PLUG] Degraded RAID

On Fri, Aug 19, 2011 at 10:37 PM, Jeff Bailey <> wrote:
> I can't find a nice, pedantic summary of what this means.  I assume that I'm
> operating without parity, and if another drive dies, I'm toast?


However, it doesn't /necessarily/ mean that a drive failed - they can
get removed for various reasons.  However, right now you are operating
in a degraded mode, which means loss of another drive will cause total
loss of the array.

Check to see if your drive is floating around somewhere and just got
dissociated for some reason.  If so you can re-add it (the array will
rebuild - possibly quickly depending on how long it was degraded).  If
the drive is failed you must of course replace it first.  Either way
once you add the drive using mdadm everything else is magic - it will
rebuild while online so you can use the underlying data seamlessly
(default is to use idle priority for the rebuild - you can force the
rebuild to go faster at a cost of disk performance if you set the
appropriate parameter under /sys).

mdadm will work as long as the new drive/partition is as large as the
old drive/partition.  If you get a large replacement drive you can
allocate the whole thing to the array (wasting the balance of space
until all the other drives get to that size and then you still need to
expand the underlying filesystem/etc).  Or, you can partition the
drive and just put a partition in the array, giving you the rest of
the drive as useful space (though non-redundant).

Typically I use the partition method, and then once I have another
drive of similar size I can mirror the extra space to get an
additional raid out of the deal.  Then I can add that raid to the
volume group if I'm using LVM and I now have a lot more space that is
still redundant.  That can get complicated and might not be a good
idea until you have a better feel for how all of this works.

Good luck!

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